Effects Of Consecutive Application Of Biochar On Nitrogen Fertilizer Transformation And Utilization In Drip-irrigated Cotton Field

Objective:It is vital to improve cotton yield and nitrogen（N）use efficiency,while decrease N fertilizer application rate for high quality development of cotton in Xinjiang.Increasing the residual N fertilizer in soil can improve N use efficiency and reduce N loss.Biochar have more potential to increase N fertilizer immobilization in soil,and elevate N use efficiency.However,the main mechanism of N fertilizer immobilization in soil after biochar application in drip-irrigated in drip-irrigated cotton field is still unclear.Basing on a long-term experiment of biochar amendment,a study was conducted to（i）analyze the effect of biochar amendment on the soil N pool composition,as well as cotton yield and apparent nitrogen recovery（ANR）;（ii）investigate the effect of biochar addition on soil N immobilization of different N fertilizer forms;（iii）determine the impact of biochar on the ¹⁵N recovery of plant and soil;（iv）explore the transformation and vertical distribution of ¹⁵N fertilizer into soil organic nitrogen（SON）fractions;（v）further pursue the relationship of between soil microbial community composition and ¹⁵N recovery of soil organic nitrogen（SON）fractions,reveal the internal mechanism of promoting the transformation of N fertilizer into SON after biochar application.The research aimed to provide a theoretical foundation for improving N use efficiency and reducing N fertilizer in the sustainable development of drip irrigation cotton fields in Xinjiang.Methods:The research consists of three experiments,namely long-term field experiment,¹⁵N tracing laboratory experiment and ¹⁵N tracing micro-plot experiment.The field experiment was conducted in 2014to 2020.Three treatments were set up as follows:（1）chemical fertilizer application alone（NF）,（2）cotton straw（CS,chemical fertilizer plus cotton straw）,（3）biochar（CB,chemical fertilizer plus cotton straw biochar）.Moreover,taking no nitrogen fertilizer and organic materials as control（CK,P and K fertilizer was applied）.This experiment focused on the effect of biochar amendment on the composition of soil N pool,as well as cotton yield and ANR.¹⁵N tracing laboratory experiment was conducted in 2019 with three soil types which collected from long-term field experiment（NF、CS、CB）and three N fertilizer forms.Three N fertilizer forms were set up as urea（amide N fertilizer,U）,ammonium sulfate（ammonium N fertilizer,AS）,potassium nitrate（nitrate N fertilizer,PN）.The N fertilizer were ¹⁵N labelled（30.3%atom enrichment）.This experiment investigated the effect of biochar amendment on the N immobilization of N fertilizer in soil.In order to determine the effect of biochar on the ¹⁵N recovery of plant and soil,and the transformation of N fertilizer into soil organic N fractions,a ¹⁵N tracing micro-plot experiment was conducted in 2019 and2020.The micro-plot was placed in the main plot of long-term field experiment（NF,CS,CB）.The ¹⁵N micro-plots received ¹⁵N labelled urea（10.5%atom enrichment）in 2019 and unlabeled urea in 2020.N rate was same as long-term field experiment.Results:（1）In the first 3 years of the experiment,soil total N（TN）in CS treatment was greater than CB treatment.From the 4^th year of the experiment,soil TN in CB treatment were significantly greater than in the NF and CS treatment.CB treatment significantly increased soil TN by 45.6%and 9.8%compared with NF and CS treatments in 2020(7^th year).There was no significant difference of cotton dry biomass,yield and ANR between CS and CB in the first 2 year,and CB treatments significantly enhanced yield and ANR compared with NF and CS treatment from the 3^rd year of the experiment.In 2019 and 2020,CB treatment significantly increased the content of soil inorganic N（IN）,and SON fractions.Compared with NF and CS,CB treatment significantly increased soil total acid hydrolysable N（AHN）by 34.9%-43.4%and 7.3%-8.4%,and increased non-hydrolysable nitrogen（NHN）by 11.3%and 14.4%,and the increase in NHN was more significant.Soil amino acid N（AAN）content in CS treatment was significantly greater than in NF and CS treatment,while other SON fractions was greatest in CB treatment.（2）After incubation experiment,the ¹⁵N recovery of N fertilizers with different forms in soil NO₃^--N was the highest,followed by SON and NH₄⁺-N.CB treatment decreased the ¹⁵N recovery in soil inorganic N pool and increased the ¹⁵N recovery in SON.The ¹⁵N recovery in SON of different ¹⁵N fertilizer increased in the order of PN<U<AS.Compared with NF and CS,CB treatment significantly increased abiotic immobilization of N fertilizer by 45.5%-81.3%and 13.2%-21.4%,increased biotic immobilization by 102%-816%and 31.0%-32.3%,respectively.The ratio of abiotic and biotic immobilization in CB treatment was less than 1,which was 28.0%-81.1%and 11.4%-20.4%less than NF and CS treatments,respectively.（3）After applying ¹⁵N fertilizer,the cumulative ¹⁵N recovery of cotton under different treatments ranged from 41.1%to 58.7%in two years.The plant ¹⁵N recovery in different treatments ranged from39.6%to 53.9%in first year and 1.5%to 4.8%in second year.CB significantly increased plant ¹⁵N recovery relative to NF and CS.The cumulative ¹⁵N recovery of plant in CB were 42.7%and 21.9%higher than in NF and CS,respectively.The soil T¹⁵N recovery in different treatments ranged from 21.9%to31.0%in 2019 and 16.1%to 24.2%in 2020.Soil T¹⁵N recovery was greatest in CB,being 41.6%-50.1%and 10.2%-18.8%greater than in NF and CS treatment.CB treatment significantly reduced ¹⁵N loss in first year and the cumulative ¹⁵N loss of two years.（4）The soil NH¹⁵N recovery was significantly higher in CB treatment than in NF and CS treatments,representing 51.9%-64.7%and 42.2%-53.7%increase in 2019-2020,respectively.CS and CB treatments significantly increased AH¹⁵N recovery in the 0-100 cm soil depth in first year,but no significant difference between CS and CB.AH¹⁵N recovery of CB treatment in the second year was 43.5%and 6.2%greater than NF and CS.Soil ¹⁵N recovery of AN,ASN in CB treatment were significantly higher than in NF and CS treatments.Compared with NF,CB significantly reduced the ratio of AH¹⁵N/T¹⁵N,and enhanced the ratio of NH¹⁵N/T¹⁵N,but CS treatment showed the opposite trend.The ratio of AS¹⁵N/T¹⁵N in CB treatment was higher than in NF and CS treatment in two years.（5）Random forest model and partial least squares path modeling results showed that AH¹⁵N recovery had significant positive effects on ¹⁵N recovery of plant and soil.Biochar amendment increased soil A¹⁵N recovery and further enhanced ¹⁵N recovery of plant and soil.Furthermore,biochar increased AS¹⁵N recovery,contributing to the reduction of N fertilizer loss.（6）Compared with NF and CS treatment,CB treatment significantly enhanced soil microbial biomass C and N,total microbial PLFA,abundance of bacteria and fungi.The High-throughput sequencing results showed that,Compared with NF,CS treatment significantly increased bacterial index of Shannon,ACE and Chao,and fungal index of ACE and Chao,while CB treatment significantly increased Shannon index of bacteria and fungi.CS and CB significantly altered the composition of bacterial and fungal communities.CS and CB notably improved the relative abundance of Acidobacteria and Chloroflexi,and the relative abundance of Acidobacteria in CB treatment was greater than in CS treatment.CS and CB treatments also enhanced the relative abundance of Ascomycota and Basidiomycota.CB treatment significantly the bacterial function of cell wall/membrane/envelope biogenesis,and amino acid transport and metabolism.SO¹⁵N recovery of was closely related to the changes of bacterial community structure.The relative abundance of RB41 and KD4.96 was positive correlation with SO¹⁵N recovery.¹⁵N recovery of AHN,NHN,AN,and ASN was positive correlation with amino acid transport and metabolism of bacterial community.Conclusion:Consecutive application of biochar significantly increased cotton yield,N use efficiency in drip irrigated cotton field.Meanwhile,the content of total N and its fractions were also increased,especially stable organic N（NHN）after biochar application.Biochar significantly increased abiotic and biotic immobilization of different N fertilizer forms,especially biotic immobilization.Biochar addition significantly enhanced ¹⁵N recovery of plant and soil,and promoted transformation of ¹⁵N fertilizer into SON fractions,especially increased ¹⁵N recovery in soil NHN,AN,and ASN,thereby reducing N fertilizer losses and improving N fertilizer use efficiency.Biochar increased soil microbial biomass,bacterial and fungal diversity.Moreover,Biochar mainly through changing bacterial community composition and amino acid transport and metabolism,thereby promoting the transformation of N fertilizer into soil organic N and increasing the retention of N fertilizer in the soil.